Since becoming the proud owner of an iPhone 3GS I’ve annoyed family, friends and colleagues silly be flashing it around and telling them which direction North is. I’ve also been marveling at the ecosystem of third party applications available (which, apparently, would cost over $140,000 if you bought them all).

But the apps – as these programs are called – that currently exist only just scratch the surface of what is going to be possible now that the iPhone knows where it is and even which direction it is pointing.

Whilst at the time of writing there are no true augmented reality applications available, there are a number in the pipeline – and their developers have not been slow to post videos showing what they can do online.

The first I came across is Nearest Tube, and app that will quite literally point you in the direction of the closest London Underground station when you hold up the iPhone. Watch the video below to see it in action.

Today I discover TwittARound (geddit), or at least a video of the first beta version. In the words of the developer, “it shows live tweets around your location on the horizon. Because of video see-through effect you see where the tweet comes from and how far it is away.” Again, seeing is believing:

So why I am suggesting that augmented reality is the next killer marketing technology? Quite simply because as these apps show, the physical and virtual worlds have just moved closer together as a result of devices like the iPhone 3GS and the ingenuity and creativity of application developers.

How long then before we have augmented reality apps that do things like:

Show messages left by others at the same location (in fact, there are map-based apps that already do this)